Perineuroma. A frequently unrecognized entity with emphasis on a plexiform variant.

AIMS

The present series describes six cases of perineurioma, a rare and frequently unrecognized entity, including one case with an unusual plexiform growth pattern.

METHODS

Retrospective clinicopathologic study of six perineuriomas.

RESULTS

All lesions occurred on the upper extremities or shoulders of adults, five in females. Histologically, all six cases of perineurioma had been initially unrecognized or misdiagnosed as dermatofibroma, fibroma of tendon sheath, neurofibroma, schwannoma, or naevus, respectively. Scanning magnification revealed well circumscribed, dermal to subcutaneous lesions without capsule formation. Besides characteristic onion bulbs", i.e. concentric whorls of epithelioid to spindle-shaped cells, there was great variation of histomorphologic features: single nodules or plexiform architecture; a few to many concentric whorls; five to several dozens of cell layers in concentric whorls; high to low cellularity; round/oval to spindle-shaped/wavy cells/nuclei; delicate to prominent collagen; variable mucin, sclerosis, and/or intralesional clefts. Immunohistochemically, all lesions were consistently positive for EMA, ultrastructurally (1 case) with evidence of perineurial differentiation such as slender and elongated, bipolar cytoplasmic processes with discontinuous basal lamina, prominent pinocytosis and desmosome-like junctions.

CONCLUSION

Our series documents that cutaneous and subcutaneous perineurioma is frequently unrecognized or misinterpreted and may occasionally show a plexiform growth pattern. The differential diagnosis of plexiform variants includes a variety of plexiform lesions such as naevi, neurofibroma, schwannoma, etc. Historically, similar plexiform lesions seem to have been published as nerve sheath myxoma/neurothekeoma, Pacinian neurofibroma or perineurial myxoma. This terminology is imprecise and confusing and, thus, should be avoided in favour of the correct term of perineurioma.